Optional
isIndicates whether the subtitle is fragmented.
The subtitle type, either "caption" or "subtitle" (default: "subtitle").
The text used to represent this track to the user (e.g. in the UI).
The language of the subtitle/caption track.
Optional
roleThe optional roles of the track.
Optional
urlThe URL to the associated file.
The id of the media track that is used to identify and set the track.